Prof Andrea GhezWe've been watching how stars at the centre of our galaxy move real 28kProf Paul DaviesThis black hole is not as big as some others real 28k

Wednesday, 20 September, 2000, 18:18 GMT 19:18 UK

Scientists look into Milky Way core

Ghez studied the motions of stars at the galaxy's core

By BBC News Online science editor Dr David Whitehouse

Astronomers have obtained the most detailed observations yet of the stars that swirl around the supermassive black hole thought to reside at our galaxy's core.

There is something quite grand in the realisation that we can expect to see the galactic centre rotate at least once in our lifetimes

Dr John Kormendy

They have allowed the scientists to pinpoint with unprecedented accuracy the hole's location, known as Sagittarius A* (Sgr A*), about 26,000 light-years from Earth.

The images were taken over a four-year period from 1995 using the Keck observatory in Hawaii.

The research is published in the journal Nature.

Galactic centre

Sagittarius A* (pronounced A-star) is located at the exact point around which our galaxy revolves.

The presence of a black hole there has been suspected for some time because of the nature of the faint radiation coming from the location and the motion of the surrounding stars.

Using the 10-metre Keck telescope atop the dormant Mauna Kea volcano on Hawaii, Professor Andrea Ghez and colleagues at the University of California, at Los Angeles, took snapshots of Sgr A*.

The short exposures "froze" atmospheric turbulence, allowing the scientists to merge the images to provide a detailed view of the galactic centre.

Enormous accelerations

This showed three stars orbiting about 16 billion km (10 billion miles) from Sgr A*. Their movement around the suspected black hole not only revealed their velocity but allowed their acceleration to be deduced.

And this gave a better analysis of the mass of the central black hole at Sgr A*, which is now estimated to be 2.6 million times that of the Sun.

"These accelerations are enormous on a galactic scale," said Dr John Kormendy, of the University of Texas at Austin, who said they would "help to identify Sagittarius A* as the supermassive black hole at the centre of the Milky Way."

Supermassive black holes are believed to be common at the cores of galaxies. In 1994, the Hubble space telescope saw evidence for such an object at the centre of the galaxy M87.

Short orbits

This was calculated to have a mass equal to two to three billion Suns and occupy a space no larger than our Solar System.

Sgr A* is nothing like as big. It would extend from the Sun to the orbit of Mars.

Astronomers point out that the three stars seen orbiting Sgr A* were accelerating so quickly that their complete orbit of the black hole may be as short as a few decades and in one case as little as 15 years.

"There is something quite grand in the realisation that we can expect, with good health and good luck, to see the galactic centre rotate at least once in our lifetimes," Dr Kormendy said.